Automatic frequency control circuit



July 24, i953 N. W. PARKER AUTOMATIC FREQUENCY CONTROL CIRCUIT 2 Sheets-Sheet l Filed Feb. 2, 1950 July 24, 1951 N. w. PARKER 2,561,817

AUTOMATIC FREQUENCY CONTROL CIRCUIT Filed Feb. 2, 1950 2 Sheets-Shea?l 2 GP/D LIMIT/NG @E620/V E E 4 BY' NORMAN w. PAR/(ER Q/ Q m V7 07W ron sys Patented July 24, 195i UNITED STATES fois-rizosffii AUTOMATIC FREQUENCYCONTBQL.

CIRCUIT Y 4 y I Norman Parker,- .Cincinnatiy 01m-io, assi-gnor'to `Aveo Manufacturing Corporation, Cincinnati;i

Ohio, a corporation. of Delaware Application Februaryi'Z, 1950Scrial .No.1'41`3942 v`lIhe present invention relates lto improvementsin automatic frequency control orrsynchronizing control systems and specifically to a novelindirect synchronizing system of particular utility in controlling the horizontal deilecting system of a televisionireceiver; The invention alsoembraces novel means andl methods forproducingcontrol pulsesV for such a system.

"'Theprior' art embraces various types of automatic synchronizing control circuits'whichA possess known advantages over directly triggered deecting systems. Three of the more popular circuits are described' in an article by E. L. Clark; Automatic FrequencyPhase Control of Television Sweep Circuits, Proceedings of the IIR; E., pp; 497 et seq'., vol. 37, No. 5, May 1949, The Institute of Radio Engineers, New York, New'York.

` The iiywheel or` synchrolock automatic frequencyv control (AFC) system utilizes a sinewave oscillator to control the discharge tube, includes a discriminator comprising a pair of diodes to compare the phases of the sine-wave signals and the-horizontalv synchronizing signals,

therebyV to derive a D. C. control potential, and applies the controlY potential to a reactance tube.

pages vr294-296-, Television Simplied, Milton Si' Kiver, second edition, D. Van Nostrand Co.,V Inc., New York, New York, 1949, and at pageS'1OI-105, Photofact Television Course, rst edition, Howard W. Sams Co., Inc., 1949, Indianapolis, Indiana.- The ilywheel system gives favorable results by comparison with directly synchronized dei'lecting systems but itutilizes ve tubes to per-- form the functionsl of horizontal discharge andj AFC control: one for sine-wave generatiomtwo for discrimination, one for reactance'control and onefor horizontal discharge. Thev flywheelsys tem employs four tubes for its AFC function.- Aprimary object of the present inventiony is -to provide an automatic synchronizing control' circuit which requires only one tube for the'performance of this function.

The sawtooth type of AFC circuit utilizes the horizontal synchronizingpulses to keyinto conductivity a pair of diodesincluded in a phase detector. This detector compares the phaseo sawtootnsignals, derived from the horizontal out--l 14. Claims. (Clt. S15-2J?) This system put, and the synchronizing pulses` therebyH -to-y developa` D; G: control potential.- `This control'potential- "is Hampliiied 'andfvapplied tol a blocking oscillator tol synchronize ther-horizontal deflectingsystem, The sawtoothsystem isdescribedjatl pages 311,9,vr S40-of the Groopublication andL pages 49'?, 498 or the Clark articleA and-in U; S; Patent' 233585545-y to Wendt; -The-sawtooth system also represents improvementover directlysyneliroy nized'systems. A- ndisadvantage and limitationllis introducedbythe-necessity-oernploying a direct adequate AFC -potential-- M The sawtooth Vsystem usesvthreel tubes for theAFG lfunction :l twodiodes forphasedetectionand'a triode or equivalent for D. C; amplication. Another-basic objeet-of-the present invention-is:toprovide asystem of high gain, without resorttoD. C. amplification, while reducing` the number ofv required` tubes.

stated-in the'Clark article, the third system, variously known as synchroguide, pulse w-idth control -or1pulsetime system uses the least number.A of tubes ofthe three systems. Synchro'guide is described at pages 83--84A of the Sams pub1ication, page 499 -of the-Clark article, and pages 3403411vorthe Grobwork. In thissystem -three diierent` voltages are applied to the grid off the control tube; thehorizontal synchronizing pulses, flyback pulses from f thev output transforrr'ier-y and parabolicpulses which are obtained by integratfA ing thesawtooth output atthe discharge capaci#y ter. The'platecurrent of then control tube'consiste-off pulses, the width'of which-fis determined by.. .the `relative position of the f synchronizing pulses atop thev peaks;I of the parabolicy pulses- The plate 'current` pulses are integrated inthe cathodecircuit. The cathode `circuit includes ya resistor uwhichis common to the grid circuit of the blockingoscillator and this current controls the frequency yof oscillation by varying', the amonntof time. required for the grid-leak bias of the oscillatorsto decline to thepoint at whichthe oscillator tube `becomes conductive. As statedfin Grobg. page,I 341, the .amplitude of-the synchronizingy pulses is critical. in determining the amount of the-control Votage, and unless very goodvlimiting-is usedv for the sync, the scanning oscillator` frequency= may vary with changes of' input signal level.l Clark statesthe limitation citi-iis prior art Asystem 'in this manner, n.499:y Thesynchronizing separator used' with the' pulse-time automatic `frequency control system, mustbe of-a type that provides synchronizing pulses of-constant amplitude; If ,the syn-- chronzingl pulse amplitude is not maintained *10. Another fundamental object of the inventionY is to provide a system for synchronizing television` apparatus which utilizes plate circuit keying of a control tube and therefore has` a higher degree of.

immunity to noise than that characteristicof prior art systems.

4 vals, which occur at line frequency, and they key the tube 49 into conductivity. The tube 49 is made to function as a grid limiter by the provision of a series grid resistor 50. By coupling means 45, 46, 5|) including this` series grid resistor, horizontal synchronizing pulses are applied with positive polarity. inthe specic eX- ample shown to thecontrol electrode of the tube 49. There is valso provided means for normally biasing the tube to vcut-off in the absence of the synchronizing pulses and flyback pulses. This means confines conductivity of the anode 'circuit of the tube tointervals of coincidence of vcorresponding ones of the synchronizing pulses A further basic object of the present invention is to provide an automatic synchronizing control i system which is rendered further insensitive to noise by the provision of grid circuit limiting.

Among the general objectsof the present invention are: to provide an improved automatic synchronizing control system which is more ef-k fective in maintaining image detail, Which is characterized by a higher signal-to-noise ratio than prior art devices, which 4has -high gain, which is more effective in preservingI the desired image raster structure inrtheface of electrical disturbances, which is more stable in its control than prior art devices, is economical in construction, and which overcomes the disadvantages and limitations which are attendant upon the use of prior art circuits such as those mentioned above.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the followingdescription of the accompanying drawings, in which .there is shown an illustrative embodiment of automatic frequency control circuit in accordancewith the invention. In the drawings:` a' Y 1 Fig. l is a circuit diag-ram generally in schematic form of a televisionyreceiver including a preferred form of automatic frequency control (AFC) circuit in accordancefwith ythe invention; Fig. 2 comprises two sets of curves employed in explaining the operation of the invention.

In accordance with one of :thefseveral appendedl claims to the preferred formrof the invention, I provide van automaticfrequency control circuit for a television pictureV tube Ideflecting system. The principal parts of this, AFC circuit are shown within the dashed outline II. The,illustrative deflecting system' hereyshowniS .oftheV type' which comprises de ecting. windings (such as-14, 75) and a signalgeneratorincluding the tubes 56 and 61) for generating deflection signals hav.- ing a frequency at leastfinrpart determinedy by a control potential appliedtheretoi (this potential being applied from` the output'of unit II to unit I2). The deflecting system. also comprises means including a vtransformer (shown Within the dashed outline I-3.)'for coupling 4the ygenerator to the deflecting windings, thereby to produce periodic beam-deecting-vwaves 'in the windings.

The function of my novelfAFC circuit is to prow vide the control potential vwhichisapplied to theA signal generator tube 5G. vvThis AFC circuit lcornprises a combination which includes `an electron' tube 49 having at least anode, cathode, and conf,

trol electrodes. I provide meansincludingna battery 5 I and fiyback pulses. This means is here shown as a The tube so operates as to generate periodic control pulses, the energy content or l width of which varies in accordance with the relative phases of the synchronizing pulses and the ilybackpulses. These controlpulses are integrated by a network 52, 53 to produce the control Vp0- tential which is known as the AFC potential and is applied to the horizontal deflecting systernlZVV to control vand synchronize the same. -Other claims are directed to the combination vof certain of the circuit elements shown within the dashed outline II, this combination functioning tovpro'e duce the control pulses. Particularv attentionl 'is invited to these features as the detailed description proceeds.

My improved automatic frequency control sys,- tein yis shown as incorporated in a televisionrreceiver which may be regarded as conventional for 4the purposes of this case except in the Arespects hereinafter set forth; This illustrative receiver'coinprises a radio frequency amplifierA stage 4 to which the input; carrier signals are ap' plied, and the following stages in cascade there; with: an oscillator modulator for convertingfthe received signals to intermediate frequency sig-k, nals (it being understood that the units 4 andi are tunable to select a desired television broadcast channel), Vfirst and second yintermediatefrequency amplifying stages 6, third and fourth intermediate-frequency amplifying stages l, a. video detector stage 8, and .anoutput unit sym--y bolically shown within the -dashed outline', 9. This receiver is of the intercar'rier sound type, as shown and described in U. S. PatentA 2,448,908 vto- Parker, and it will be understood that separation of the video and sound signals occurs Within:V the .output unit Sfwhich includes a videoampliV fier stage I6, a grounded-gridtype D. C. restorer and sync kseparator-tube 28, and a cathode' ray. picture tube A25.

The elements collectively indicated vwithin the dashed outline 9 are purely conventional, as are: likewise the units '4, 5, 6,-1 and 8, so that further descriptionythereof is deemed' unnecessary here-l in. Briefly,.l^ lowever,v the detector output is cou pled to the control electrode of video` amplifier tube I6 as by parallel combination of an inductor; I4 anda resistor I5. -Tube If is provided witha, cathode resistor Il.y The screenof tube I6 is by'` passed by a capacitor I8 and the anode' is 'cou. I

pled through a seriespeaking circuit (compris-1 ing .a parallel combination-of' an inductor 20' and av damping resistor ZI) and aresonant circuit (comprising a v parallel -combination of an in, ductor 22 and a capacitor23.) and aj coupling ca; -pacitor 24 tothe' grid of-a Icathoderay picture tube 25.y The anode of the'video amplifier hasxaloadfcomprisingfa seriescombination of van in-j'- ductor 2G and aresistorwlfleading to a source of space lcurrent ,(not; shown) The symbol .-I-,By is:

hereimto indicatey various. anode potential sources,l .which need-.not-- be the same. The'direct currentreinsertingi circuitfcomprises a grounded.- griditrio dephaving its cathode.Y coupled to the lower end of coil 2t through a capacitor-:Hand a resistor 3%. and having its plate connectedl to one end of resistors? through a resistor 3|. AThe cathode-anode circuit of Y triode`28 is' shuntedl by a-.series combination of. resistors l2-and. 33. As.y is well'knownto those, skilled in. the art, agdirect current component which is a measurezof the over-all. illumination appears at' the cathode of tubeZ-Sqandfis applied to i' the grid of the'Y picture tube through aresistor 3-4. f D. C. restorerinetmor-ks. of this general: characterare shown.. and. described. in anumber. of publicationsincluding ELS'. Patent 2,240,28l issued.to R. C. Ballard on April. 29 1941. Separation between video and sound signals occurs byreason of circuitry including. element 3%', a sound take-off resonant circuit (not shown)` being magnetically coupled. to. inductor 22as. a. pri-mary. Tube andthecircuit elementsim.- mediately associated therewith separate the sync signal components Vfrom the videoA ,signal components. The plate circuit. of tube28 is coupled. to a lsy nchronizing signal amplier unit of conven.- tional character,v illustratedA within the dashed outline l0, as by a coupling capacitorLSS.- t

The separated horizontal synchronizingrsig-nals. areapplied to the input of the novel automatic frequency control circuit provided in accordance with the present invention. The unit l0, which isfper se Conventional. performs the function of. amplifying` the vertical and' horizontal. synchro-k nizingsignals and further separating them from. the. video signal components, preparatory to the application of the synchronizing pulses to my novel circuit, shown within thed'ashed outline! l.. It'will be understood that any suitable inter-sync separator or. amplifier circuit may be providedin lieu oi" the circuit herein shown within the dashed' outlines It; This specic synchronizing signal amplier circuit need not be describedY in detaill. Bri'ey, however, it comprises a pairv of triode tubes fr? and 38. The input circuit of tube 3T is coupled totheanode ofthe-sync'separator tubeI 2181' as-byacoupling network including a capaci'- tor 35 andl al grid resistor 43131, and grid-recticationA restorationof the D. C. component of the syn'- chronizing signals occurs-within unit I-. The D; `C'..restoration action also provides self-bias for tubel 31. rllhe anode of. tube-31 is coupled to al suitable source of. space current (not shown)`- through: a resistor di and the anode of tube 38l iselikewise connected. toathat source.'4 The output circuit oiftube 37 is coupled to theinput-circuit of. tube 381' by a conventional resistance-capaci:-

tance coupling network comprising the elements* Sil'andili. Tube 3.l,.in.addition.to its amplifying function, being. driven near cut-ofi by the-peaks ot negative input pulses, clips black or negativegoingnoise. Tube functions.asanimpedancetransformation device or cathode follower. In-v eluded in its cathode circuit. is avlter network` comprising a. parallel combination of. resistor i121A and.. capacitor 43.

' 6 chronizing pulses may be` separated outas by dif-f ferentiation. and separately applied if desired; The conductor 4l from unit l0 is appliedto a dou.- ble integration network (not shown), for exam-l ple, for purposes of separating out the vertical synchronizing pulses and the latterY alone are utilizedstocontrol the vertical dei'lectingsystem (not shown).

Anautomatic gain. control circuiti@ may optionally'be used. The nature of this gain control device is not material to the description of' thev present invention. The detailsof a suitabledeviceherein suggestedY by the blockv 138 and asso@ ciated. circuit connections are fully disclosed in the'U. S. patent application of Parker and Wissel, entitled Automatic Gain Control, Serial No. 10%1763 led in the; United States Patent Oice, on June 30, 1949; Reference may be made to that application for a. specific description of the automatic gaincontrol herein suggested.

The-units 9', Ili and I2, .herein described brieiiy, are included in this disclosure in order to showt (l) the application of suitable synchronizingim'- pulses, to the input of my novel .circuit i l; (Z-)thei application of retrace pulses to mynovel circuit. |.I ;A (f3) theuse of the direct current control. po-l tential provided. by my novel circuit l I to control;

Within intervals. These pulses are' obtained Ifrom ail` winding 35 which. is included in the horizontal output.Y transformer assembly, shownwithin the rectangular dashed outline 13. The manner inA which the retrace pulses are developed willzube explained in some detail. It Will be understood. that the'ultimate'function of my novel: automatic. frequency control circuit, included within the'f dashed.v outline H, is to apply to the horizontal? deflecting system (shown within the dashedvoutline. I2)` .adirect current control potential of such a` magnitude as to maintain synchronism (i. e. a. predetermined desired phase relationship between the. synchronizing signals and the flyback' pulses).

The horizontalv delecting system which is; shown. with-in. the'. dashed outline i2 is conVen-.l tional and. need not be described in detail herein. Bri'ey, however, unit i2 comprises the following. principal components: a blocking oscillator andY discharge; tube 56,. apower. outputk tube El', a pr'i.A

mary. 'I0 anda secondary 11|. of anoutput trans`y a tap onthe auto-transformer and ground. A'

resonant. circuit'comprising aparallelcombina# tion of an inductor5'9, a capacitor 6D and a dampingresistor 6-I is interposedlbetween this tap andf the high potential terminal of capacitor 58. Plate voltage is supplied to theA blocking oscillator thr'oug'hav circuit comprising conductor 62, drop'- pingl resistor S3, tap- 64, and a part of transforme' f er 51'. The sawtooth voltages employed for hori' l zontal-deflection are developed across discharge capacitor 58, tube 56v functioning not only as a blocking oscillator tubebut also as a discharge tube; Theblocking rate or frequency of oscilia' arenal?.

tion of tube B isvcontrolled by causing the dis.- charge of capacitor 55 to reach the cut-off point at a variable time. controlled by the AFC potential output of unit l l. v v The discharge capacitor 58 is coupled, as by a capacitor 65 and a grid resistor 66 to the gridof a horizontal output amplifier tube Bl, the latter being provided with a cathode resistor .68, -bypassed by a capacitor. B9. rIhe' output of this amplifier stage is'ccupled by a transformer network 10,7! to the deilecting coils 14, l5 and the current waves appearing in the plate circuit of this amplier tube arey employed to produce periodically recurring sawtooth .currents of line fre'- quency in these coils, rthereby to deflect theelectronic beam in the picture tube at line-frequency.

The system intercoupling the horizontal output tube and the deflection yoke will be under.- stood by reference to the following patents and publications: Kiver, Television Simplified, pages 207-213, Second Edition, 1948, D. Van Nostrand,

Inc., New York; U. S. Patent No. 2,440,418, Tourshou. Reference to those publications is made for a detailed description of the network. Briey, the primary 'I0 of the horizontalfoutput transformer is magnetically coupled to a secondary 1I comprising series portions 12 and 13,-of. which portion 'I3 is vcoupled across the horizontal deflecting coils 14, 15 of the yoke circuit. These coils form part of a yoke assembly (not shown) encircling the neck of the cathode ray image reproducing tube 25.

The voltage variations applied to the control electrode of power tube B1 produce a rising plate current in this tube during scansion, which current is cut off at the beginning of retrace time.' The current in the deflection coils 14, 15 and the horizontal output transformer does not disappear at the Ainstant of cut-off of tube 6l, however, due to the inherent distributed capacity of the circuit. The inductance of these coils and the transformer,'together with the above-mentioned distributed capacity, forms a tuned cir-l tinues to conduct` until tube 6l again becomes' conductive. The voltage developed across capacitor .l1 is such as to increase or .boost the voltage of the D. C. power source (not shown),

connected to terminal 1B." It will be noted that the anode potentialsupply for tube 56 is completed to this terminal I8 through a circuit comprising resistor 19 and line 62.

The primary winding of the horizontal output transformer is connected to a capacitor 8| in such a manner that a varying voltage is ,developed across capacitor 8l, which voltage is applied to capacitor ll through an inductor 82 for purposes of linearity control, as explained in the aforementioned Tourshou patent. v

For purposes of lwidth control, there is conHV nected in shunt with portion of secondary an inductor 83. The operation of the width-control is well known to those skilled in the art and is considered in such publications as U. S. Patent 2,449,969 to Anthony Wright.

When the quick collapse of plate current through tube El occurs, at the end of trace, there si isv producedacross winding l0, by reason of selfinduction,` av high voltage pulse. One of pulses occurs-during each retrace interval. They are known as fflyback pulses and they occur at linedeection rate. v v

' 'The elements beginning with the reference numeral 56C and ending lwith the reference numeral 83 are herein shown for the purpose of disclosing-an illustrative source of ilyback or keying or retrace or deflection-rate pulses employed in the automatic frequency control system in accordance with the invention. The retrace pulse sourcedesignated bythe reference numeralsbeginning withv 56 and ending with 83, such elements" being showny within the dashed outline l2, is illustrative, and other arrangements for deriving deflection-rate. or retrace pulses from the horizontal deflecting system may be employed within the scope of the claims appended hereto. v Mynovel AFC circuit comprises a tube 49, here illustratively shown as a triode, having grid 90, cathode 9i and anode 92 electrodes. Means compris'ng a coupling capacitor 45, a series resistor 5DA and a grid resistor 46, the latter being connected vbetween ground and the junction of elements .45, 50, is provided for applying the horizontal synchronizing or composite synchronizing pulses from the output of unit l0 to the gridcathode circuit of tube 4S. Biasing means, here shown as a battery 5l in thecathode circuit with positive terminal connected to cathode, is pro-` vided for rendering the tube 49 nonconductive inthe absence of synchronizing pulses, eveny when positive keying pulses are applied to its anode circuit. This biasing means renders the cathode normallyr positive relative to the grid. IThe grid-cathode circuit is employed as a limiter circuit by the insertion. of series resistor 50, which is very large compared to the grid-tocathode resistance when grid current flows. Applied positive pulses at the grid must drive the grid positive by an amount equal to the value of the bias (imposed by battery 5|) before the biasing effect is overcome. A further rise in the input voltage 'produces grid current which results in` limiting the voltage at the-grid. The

series resistor prevents the grid from going appreciably above the cathode potential. The grid-to-cathode resistance drops from an innite value, when the grid is negative relative to the cathode, to a small Value, say on the order' of y1000 ohms, when the grid attempts to become positive relative to the cathode. When a series'` resistor 50 on the order of, say, 100,0007ohms, isl placed in series with the grid, the drop acrossv the grid-cathode resistance is negligible com-1 pared to that which is developed across the resister-50.

kAssuming application of the keying pulses to the plate, the tube 49 is biased to cut-off at al level preferably at or slightly above the pedestal height and below the peaks of the superimposed synchronizing pulse signals. When a synchronizing signal pulse is applied to tube 494v the grid attempts to swing positive, but grid current flows through the resistor 50, developing a.

drop across resistor 50 may be considered as an..

automatic bias developed during the fsynchroe` nizing signal pulse intervals. The grid circuit oftube 49causes the tube to function not only as an amplilier but valso-asa limiteryso 'that vtwo significant advantages accrue; First, my AFC circuit is not'dependent ion a critical predetermined amplitude of the-applied synchronizing -pulseasince it performs own limiting function; second, vmy AFC circultp as herein shown in detail, vpossesses a `high .de-

gree of immunity to black noisesuperimposedon the synchronizing signal pulses, such'l noise pulses being removed bythelimiting action@v Another significant advantage of my AFCcircuit is that the control-tube'iskeyedfintoaconductivity at the plate circuit, -the data ,pulses` from the synchronizing signal unit 'i0 andthe yback pulses from the output of the horizontal deii-ecting circuitbeing separately applied/to the grid .and plate circuits, respectively. Since the tube t9' is vconductiveV only ,during intervals of coincidence between horizontal `sync pulses and yback pulses, vit is not .responsive tovvertical pulses or videoinformatifon, so that composite horizontal and vertical :sync may'beappliecltov unit H. For the purpose of ,providing keying pulses there'is provided an :additional `winding v35 y in the output transformer unit i3, magnetically coupled to primaryand secondary ll.

rOne portion 94 ofthis'ylinding is in'` series with an integrating circuitcomprising :aresistor' 52- and capacitor 53. vTheelernents 52 VAand -53zare connected in parallel 4between `oneA terminalfi. e. the :center itap) cf-portion 94 (ofwinding) .and ground. y'line-,other terminalof portion -94` is connected to I`anode 9210i tube 39; The yinput admittance roi tube 49 is .neutralized :by a v,capacitor'196'whiohcouplespthe plate and grid `circuits offtubeii in such .a-Wayrthat the current passing through `it is :ofthe proper amplitude and phase tofneutralize the fe-ect of the current flowing via'fthe grid-*plate capacitance. The voltage at the end of Winding portion 95, vconnected to capacitor 9B, is-inphase opposition tothe voltage' at'itheend y.of winding portion 94, connected pto 1 the plate of tube The voltage across the neutralizing inductance295 .causes the grid of tube 49; connected Eto .capacitor-.96, to receive a current which neutralizes energy transfer through .the tube capacitance, according to principles eX- plained'pby Terman, Radio Engineering, pp. ',367 et '-seq., third edition, 11947., McGraw-Hill Book Co., Inc., .New York, New York.

Winding AVportion 94,' `being Yin series with the plate'circuit v`oi' vtube y59, serves .as the rimmediate source-.ofpositive fl-yback pulses (developed .dur-

ing retrace), of vline frequency, which Y:periodicallyv key- :triode into conductivity ll'his 'tube :platerecties into :the time .constant integrating network comprising resistor-.52 and zcapacitor'53..

kTl'ieitirne constantof '.this network'52, E5-.must

circuit to oscillateand effectively prevents unde' sired oscillation because it reduces the,alternat ing current vgain in the region of natural reso-` nance of the loop circuit, it being un'derstood'tha'tv all systems of automatic frequency control which depend upon''the derivation of va control signal from signals corresponding to order and response' are inherentlyv capable'of oscillation andwill os''y cillate at the natural resonance frequency unless precautions .are taken to suppress such 'oscilla' tions. The theory of ani-@hunting networks such". asthat which comprises felernentsi, Ii9'issw`ellv known toy the artv and need not be further cle-4` soribed herein. Thegridcircuitof the blocking" oscillator tube 50, vwhich includes capacity 5,5,is completed by thev series combination of gridfre'-l sistor l0() and resistor r52, resistor |00 being con-"E nected between the grid of tube 56 and the ,juno-"I tion of the integrating network-52, 53, and resistor95.l The anti-hunting network may -also'bef regarded as an error-rate circuit, as explained` incolumns 2 and v3, page 60,'Radio and Televi` sion News, January 1950, vol. .43, No. 1,published by zonnavis Publishing' oo., 135 No. Wabash Avenue, Chicago 1,1Illinois.

bezlongwtco'mpared to the. interval between :yback pulses. This ,network-.functions as 1av low pass filter which preferably should Ypass frequencies on vfthe order of 30 icycles or less. Connected acrossthe integrating network is a-series combination `Aof resistor '99 :and .a capacitor 99,-fwhich functions asfran anti-:hunting circuit. Theantihunt circuitccrnprisingresistor 98.' and capacitor SQ-iSQrQportiOned-rin the following illustrative` In order to reduce the gain of the feedback 10913 while I do nordesire ,to be unduly limited t9 any specic set of ,circuitfparametera the follows-' ing parameters which have 'been found.v t0 .be entirely satisfactory arejsubmittejd by way Oflf lustration and not by limitation:

Tube es, one-half section of Type ssN'z, orcos ResistorZ, 6800 ohms Capacitor 113, 20 micromicrofarads Capacitor 45, .01 microfarad Resistor 46,110,000 ohms l Resistor. 50, 100,000 ohms 1 Tube 49,. one-half:sectionof'Type 6SN'7, or' 6.15 Capacitor 96, 1-5 ymicromicrofarads Resistor 52, 150,000 ohms Capacitor 53, .02 microfarad Resistor 93, 10,000 ohms Capacitor 99, .25 microfarad Resistor |00, li'70,000 ohms Tube .55, one-halflsection lof Type .,6SN7, or 6J5 Coming now to the operation of `the invention, y

it will be assumed that synchronizing ypulses Jnf as represented by curve A, in Fig.,2.` it will also be assumed that flyback pulses voi positive polarity are applied to the plate circutof tube/49j.

These pulses are ideally represented by curve B oflFig. 2, although it ,will be understood that',l in actual practice the keying pulses are actually curved at the top. The grid circuit of tube`49l is so biased by source`l5l that only the super-A imposed synchronizing pulses are in the .conduce 'y tive region. In thespecii'lc embodiment illustratled wherein positive pulses ,are applied to the grid of tube 99, the blocking oscillator has ,a free running frequency which, is greater ,than that Hof the synchronizing pulses, i. e., greater .than line-fh, frequency. In theabsence ofan ,AFC .output from v unit Il, `in other words, Ithis blocking yoscillator',1 would tend to run-at a `more .rapid rate .than

ilection rate. A negative control potential is thereforerequired to effect synchronismand the condition necessary to accomplish this result is that the initiation ofthe keying pulse occur at or later than the initiationv of thev synchronizing pulse. This is shown in the curves in Fig. 2, to being the instant of initiation of a synchronizingpulse and t1 being the instant of an initiation 'of a keying pulse. Between instants t1 and t2', the latter being the instant at which the syn.- chronizing pulse ceases or terminates, the synchronizing pulse and the corresponding keying pulse are coincident, and the interval between ti' and tz is known as the interval of coincidence. The difference between ordery and response, i. e., the difference between the phase of the synchronizing signals and the phase of the flyback signals is functionally related to and proportional tothe time of coincidence. The resultant output plate current pulse of tube '49, as shown in curvey C of Fig. 2, has a total energy content or Width or duration which is dependent on the in terval ofcoincidence. The energy content of this pulse is therefore a measure of the amount by whichA the flyback pulses follow the synchronizing pulses in phase.v Being a measure of that deviation, it is also a measure of the corrective po-y tential which is required, when applied to thev blocking oscillator tube 56, to restore phase coin-` cidence. When the flybackpulses are in rate synchronism with the synchronizing pulses, each current pulse C assumes a predetermined value, there being a predetermined phase displacement between the two types of pulses. H'IIhe direct current potential which is applied to the grid circuit of tube 56 is obtained by .in-- tegrating or averaging a large number of currentpulses such as those shown in curve C of Fig'. 2, this function being performed by the integrating network '52, 53 into which tube 49 plate rectifies. The function of the integrating network is to convert a plurality of such pulses into a relatively steady control potential illustrated by the curve D (Fig. 2).

Under normal conditions the blocking oscillator tube becomes conductive at a time slightly later-than the arrival of a'synchronizing pulse and therefore a certain bias is normally developed across the integrating network 52, 53. This value corresponds to that for which rthe frequency of the blocking oscillator is equal to the frequency of the synchronizing signals and the two signals. are therefore substantially in synchronism and' in the desired phase relationship. The systemv is controlled for the reason that if the blocking oscillator tends to accelerate there is a decrease in the phase difference between the synchronizwith the effect that the frequency of the blocking oscillator increases in such a Way as to restore synchronism. p

It will be observed that tube 49 is essentially a sampling` device. It is keyed into conductivity by each flyback pulses in order to sample that portion of the corresponding synchronizing pulse which is coincident with the keying pulse.

The

AFCcontrol I potential 'is not ar function yof one sampling operation but in fact is a function of 'a' plurality of sampling operations, so that the' circuit enjoys the advantages of indirect synchrol nization and the immunity to'noise which charac-l terizes AFCv devices which are energy-responsive; The circuit rather than amplitude responsive. enjoys a further immunity from noise in that the advantages of limiting are achieved, the grid cir-'v cuit of tube 49 being so arranged, particularly by virtue vof the presence of series resistor 50, as to; cutoff any noise pulses which may be superim.- posed on the synchronizing pulses. A still further;

immunity from noise is'insured by reason of the fact that the tube 49 is platel keyed intov operation and is conductive only during the intervals'f The tube is so biased by the.

of coincidence. source 5l that it is not conductive, even in the chronism, the natural free running period of the .blocking oscillator being such'V that thev frequency; of'its output pulses tends to be greater than they frequency of the synchronizing pulses.

When the specific system herein disclosed is in 'l synchronism a, negative AFC' potential of a predetermined value is applied to blocking oscilla--l tor 56 by unit Il. The system operates in such a way that when there is a departure from synchronism caused by a slowing down of the blocking os-l cillator the intervals of coincidence decrease andV the control potential becomes less negative. When an increasing negative potential is required to bring an oscillator from a free-running con#- dition into synchronismit follows that the naturali free-running frequency of the oscillator is higherl than that of the synchronizing signals, There'y fore, `when synchronizing signals of positive po-'f' larity are applied to the grid circuit of tube 49,/ the constants of the blocking oscillator circuit (tube 56 and associated elements) are so determined that the free running frequency of the osl cillator is higher than the horizontal line fre-` quency. This results in an additional advantage in that, when the receiver is de-tuned from an available signal channel, the blocking oscillator 56 Acan be designed to run at a frequency above l' the audible frequency range, so that it does not constitute a, source of audible interference.v

It will of course be understood that it is Within f' the spirit'of the present invention and the scope of the claims appended hereto to employ any type of deflection'generator and the invention is not limited to utility with a blocking oscillator such as that shown herein but may be used with any 5 deflection signal generator of the type which is" controlled as to frequency as by a direct current potential, for example, a multivibrator.

synchronizing signals of positive polarity are apnals.

When t a multivibrator of the tube which is controlled by'f a negative control potential is employed and when lf3" Y #It'swill furtner be 'understood ffthat the .come positef V*syrnchronizing 'signals .inclusive ref both horizontalH-:and"vertical signals-may :'.be 'a'pplied tothe-tubefil, -lfe'causet'hev tubelis so'bi'ase'd 'that it'sis conductive only .during thefabove-mentioned intervals =o-roincidence, i and fit 'hasthe advantage, therefore', .of .n-ot being responsive 'to'.'the vertical synchronizing `signal Acomponents ofthe 'composite signal. SIhereiorefthereis n'o necessity -tohprecede .tube'il .circuits which entirely .remove vertical information' fromY the horizontal 'sym chronzing signals. The'tube'ibeing platekeyed andconduct'rve only dnringthe'interva'ls offscoincidence, lit is not afctedbyfany videoconten't which may-@pass'throughithe'lunits' and lin-ano.' be iapplied*to vthe tubed'.'

'I-'he'#embodimenthereinabovedescribed in de# tail lso operates ltodevelop periodic plate pulses in lt'ube'iii which increases v-inwvidth fa's `synchro-- tracep'ulse keys thetube '-'49v` into conductivity during "that'part of' the corresponding synchronizinfg pulse `in-te'rvalfwhiohcoinciiies in ftimewith the retrace pulse. In -eiec'a the tube 4%Fsample's the '-*w'idth y of Athe -synchronizingfpulse during the periodbf coincidence In `jthissystem the control potential becomes more negative as synchronismis:approached and therefore theblocking oscillator has a -ir'ee-r-unning'- frequency greater than 4line"frequ'ency.

f ""Ilie invention also 'contemplates the reverseop-4 erationfwherein the Lsynchronizing pulses are 1 applied"` tov` the -control tube '5('suchras 9) with nega-` tive `polarity. In that case 'Ithe'control potential becomes lless `negative `as -synchron'ism is Papproachedland ablocking oscilla-tori having a `freerunn'ing-#ireefuency yless thanlineeirequency is employed.' YThese conditions :are'illustrated iin' Fig. 2""(ighthands`ide), curve-'A1, showingfassync pulse, curve B1, `the corresponding re'trace'pulse,r

curve C1, the resultant periodic. plate `p'ulseo tube B2 and curve-D1 the`D. C. lcontrol potential.

The control tube is then lsc biased'that thefsy'nechronizing .pulses'cut `o'tlthe tube, which is :conductive' only `during"fthat portion Aof a positive plate-pulse 'interval which is -not'coincidfent with thecerre'sponding *synchronizing pulsei'intervfall.` Such'fasystem samples the width of fthezretrace' ptilse dur-ing 'the periodoi non-coincidence. As

coincidence increases,"-in'such a system; ithe width oitlie plate pulse `decreases and the controlipotentialbecomes 'less negative. Black. noise superimposed Ion the synchronizing' pulses is if clipped in l this system because the synchronizing peaks extend 'to -or ybeyond fthe cutoi regionl Grid circuit limiting is not then -provided'nnd yvertical synchronizing signals must 'be separated outibe-y for-e Avap'p'limttion ni horizontal synchronizing ptsesto'the controlledtube.

Whiletlierefhas beenshovvnand desoribedwhat y is'ati present 'consideredtoribe the vpreferred embodiment of the vvpresent'l-inv-enti'on, it vwill be obn viens 'tto tliose "skilled-1in k'the art'rthat yvarious changes `and fnmodications f may l be Vmade "Within i ing la-ma'gnitude dependent :on 'the :extent of rde-y viation iromsynchronism, andzelectron-bea'm'de il'e'cting (means includinga transformer for cou'- pling said `generator to' said .deilecting `windings whereby to produce :periodic `beam-deilecting Waves' therein-'comprising thefcombination of an electron-tube lhaving at leastV anode, cathode, and.

synchronizing pulses, means in circuit withsaid anode .for integrating Vsaidresultant pulses yinto said negative .control potential, and means "for n applying saidpo'tential to said blocking-oscillator to maintain .synchronisinY said blocking-.oscillator having a free running frequency greater than 'that of said synchronizing pulses so `that'the, development of some control potential is required "to maintain synchronism.

2.v An automatic frequency control circuit fior` an indirectly synchronized'television picturetube" deflecting systemsaid system being of` the type; which. comprises beam-deflecting elements and' a signal `generator for ,generating deection sig-v nals having la frequency at leastin ,part deter--` mined by a xcontrol potential, .and means for coupling .said generator to said .deiiecting elements, whereby `to `produce `.periodic beam-de- Y retrace pulses Aof positive polarity, .thereby .to

key said tube into conductivity, means .for ap.

plying horizontal synchronizing ,pulses of ,positive polarity .to-,said .contol-electrode,` whereby .said

tube .generates periodic resultant pulses, the

energy .content of .which varies with the relative` phases ofsaidsynchronizingpulses and said retrace pulses, .m eansfor integrating. said resultant' pulses :intosaid-control potential, and means for applying said .potential to said generator to maintain synchronism.

. .3 In .an automatic ffrequencycontrol .circuit .foran indirectly synchronizedpicture tube deilecting systemfsaidsystem being of .the type whichcomprises .deilecting windings anda signal v.gen-- erator .for ,generating la .deflection .signal `having a frequency 'at .leastinipart determinedby acontrol. potential applied thereto, electron-.beam del fiectingrmeans -includinga transformer for .cou-

plingsaidgenerator .to .said .deflecting windingsk whereby .to .produce periodic .beam-,deflecting'- erator `to maintainsynchronism, anda .source of horizontal `synchronizing signals of Vpositive ,po-

larity-the improvement which resides in a novelI circuit for generating .said .periodic ypulses .com-

prising the combination foi Yan electron tube havingat :least anode, cathode, and fcontrolzelectrdes, means including a Winding in` said transf" former-for. applying positive keying pulses at de'- ection rateto the anode-cathode circuit of saidtube, means for applying said synchronizing v pulsesfto the control electrode-cathode circuit of said-ftube, and means for biasing said tube to cut-oiexceptduring periods offcoincidence ci?A corresponding ones of said synchronizing pulses and-,said keying pulses. f l ,54. In an automatic frequency control circuit for an indirectly synchronized picture tube delecting system-said system being of the vtype Whichf'com'prises deflecting elements and a signal generator for generating a deflection signal having a frequency at least in part determinedv by' acontrol potential applied thereto, electronbeam deflecting' means for coupling said generatorito said elements whereby to produce periodic beam-deflecting signals thereat, means for integrating periodic pulses into said controlpotential, meansfor applying said control potential to said signal generator to maintain synohronism, and asource ofA `horizontal synchronizing vsignals of .positive ,polarity-the improvement which resides in a novel circuit `for generating said periodicfpulses comprising the combination of an electron tube having at least anode, cathode, and control electrodes, means for applying positive keying pulses at deflection rate to the anodecathode circuit of said tube, means including a coupling capacitor and a shunt resistor and a series grid-current limiting resistor for applying said synchronizing pulses to the rcontrol electrode-f cathode circuit of said tube, and means for biasing said tube to cut-off except during periods of coincidence or corresponding ones of said synchronizing pulses and said keying pulses. V

5. In a television .receiver AFC system, la circuit ior generating periodic control pulses comprising, in combination, a source of positive re trace pulses, a source of synchronizing pulses, a single tube vhaving at least anode and cathode and control electrodes, means for applying'said positive, retrace pulses to the anode-cathode circuitfrofsaid tube, and means for applying said synchronizing pulses to the cathode-control electrode circuit of said tube.

. vE5.fIn,.a television receiver AFC system, a cir cuit for' generating periodic pulses having a duration proportional to the phase displacement between synchronizing pulses and deflection-rate pulses comprising, in combination, a source'of pcsitivcdeflection-rate pulses, a source of synchronizing pulses, a single tube having at least' anode and cathode and control electrodes, means for applying said positive deflection-rate pulses to the anode-cathode circuit of said tube, and means for applying said synchronizing pulses, to

the cathode-control electrode circuit of saidvtube.

` 7, `In a television receiver AFC system, a circuit for generating periodic pulses having a time duration proportional to the phase displacement between synchronizing pulses .and deflection-rate pulses comprising, in combination, a source of positive deection-rate pulses, a source of positive synchronizing pulses, a tube having at least anode and cathode and control electrodes, means for applying said positive deflection-rate pulses to the anode-cathode circuit of said tube, and means for applying said positive synchronizing pulses to the cathode-control electrode circuit of said tube.

y8. In a television receiver AFC system, a cir-4 cuit for generating periodic pulses having a Width proportional to the phase displacement between synchronizing pulses and deflection-rate pulses comprising, in combination, a sourceof positive deiiection-rate pulses, 'a source ofy positive syn chronizing pulses, a single limiter-amplifier tube having at least anode and cathode and rcontrol electrodes, means for applying said positivede` ilection-rate pulses to the anode-cathode circuit of said tube, and means including a series control electrode-current limiting resistor for applying said synchronizing pulses to the cathode-control electrode circuit of lsaid tube.

9. In a television receiver AFC-system, a cir-- circuit for generating periodic pulses having an energy content proportional to the phase displacement between synchronizing pulses and de` ilection-rate pulses comprising, in combination; a source of positive deflection-rate pulses,` a. source of positive synchronizing pulses, a vacuumf tube Vhaving at least anode and cathode` and control electrodes, means for applying `said' positive deflection-rate pulses to the anode-cathode circuit of said tube, means for applying said synchronizing pulses to the cathode-control electrode circuit of said tube, and biasing means for conning conductivity of the anode-cathode circuit of said tube to periods of coincidence of corresponding ones of said synchronizing pulses and said deflection-rate pulses.

k10. In a television receiver horizontal AE'Csys-v tem, a circuit for generating periodic pulses having an energy content proportional to the phase displacement between synchronizing pulses and line-frequency keying pulses comprising, in combination, a source of positive keying pulses, a source of positive synchronizing pulses, a single limiter-amplier tube having at least anode, and cathode and control electrodes, means for applying said positive keying pulses to the anode-cathode circuit of said tube, means for applyingsaid synchronizing pulses to the cathode-controlelectrode circuit of said tube. y i

11. In a television receiver AFC system, a cir-- cuit for generating periodic pulses having anenergy content proportional to the phase displacement between synchronizing pulses and line-frequency pulses comprising, in combina-l tiona source of positive line-frequency pulses, a source of negative synchronizing pulses, a. tube having at least anode and cathode and.l control electrodes, means for .applying said linefrequency pulses to the anode-cathode circuit l of said tube, means for applying said synchronizing pulses tothe cathode-control electrode circuit: of said tube, and biasingV means forl confining conductivity of said tube to those'portions of the. line-frequency pulse intervals which are not coincident with said synchronizing pulses.

12. In a television receiver AFC system, a circuit for generating periodic pulses having an energy content proportional to the' phase displacement between synchronizing pulses and key-v ing pulses comprising,` in combination, a source of positive keying pulses, a vsource of synchronize ing pulses, a vacuum tube having at least anode and cathode and control electrode, and means. for separately applying said keying kpulses and said synchronizing pulses to the output and input circuits of said'tube.

13. In a television receiver, the combination of a line-deflecting system of the type which is frcy quency-controlled by a unidirectional control potential, and a control circuit for developing said potential, said control circuit comprising a vac# uum'tube having at least anode, cathode, and;

751 control electrodes, means coupling an outp'ut cir-l l? cuit of said line-derlecting system to the anode circuit off said tube 4to periodically apply positive keying pulses to said anode, means for applying synchronizing pulses to the input circuit of said` l pulses,means for integrating said resultant pulses into said control potential representative of the phase diierence between said synchronizing pulses and said keying pulses, and means for applying said potential to said line-delecting system to stabilize said phase difference.

14. In a television receiver, the combination of a line-denectng system of the type including a blocking oscillator which has a free-running frequency greater than the frequency at synchronism and is frequency-controlled by a negative unidirectional control potential, and a control circuit for developing said potential, said control circuit comprising a grid-circuit limiter including a vacuum tube having at least anode, cathode, and control electrodes, means coupling an output circuit of said line deecting system to the anode circuit of said tube to periodically apply positive keying pulses to said anode, means for applying positive synchronizing pulses to the input circuit of said tube comprised of said cathode and control electrode, means for rendering said tube l nonconductive except during each interval ofcoincidence of a keying pulse and a synchronizing pulse, whereby said tube generates resultant periodic pulses, means for integrating said resultant pulses into said control potential representative of the phase diierencebetween said synchronizing pulses and said control pulses, and means for applying said potential to said blocking oscillator to stabilize said phase difference.

NORMAN W. PARKER.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Great Britain Feb. 14, 1938 

